Conventional quadrupole ion trap technology has been developed and practically used for several decades. Literature and patents about this technique is well recorded in the book “Practical Aspects of Ion Trap Mass Spectrometry” edited by R. E. March and J. F. J. Todd. As another approach for driving a quadrupole or, in general, a hyperboloid mass spectrometer, E. P. Sheretov employed a pulse generator to feed the ion trap with the rectangular wave voltage. With this method ions can also be stored and sorted according to their mass-to-charge ratios. Publications about this study date back to the 1970's and the paper titled “Base of the theory of quadrupole mass spectrometers during pulse feeding” (referred to hereinafter as paper 1) by E. P. Sheretov et al published in J. Tech. Phys 42 (1972) gives the fundamental theory of this technique. Because of the flexibility in applying a rectangular wave voltage rather than a sinusoidal harmonic voltage, and also the advances in digital and switching electronic circuitry, this rectangular wave driving technique appeals to the modern concept of instrumentation in mass analysis. Besides the old fashioned mass selective storage mode which is basically suitable for low mass residual gas analysis, work on mass selective instability mode in which ions are scanned through the boundary of the well known “a-q” stability diagram and sequentially ejected and detected has also been reported. In PCT Patent Application No. GB00/03964 there is disclosed a method of mass analysis whereby a rectangular wave voltage is supplied to the ring electrode of a quadrupole ion trap, and further dipole excitation voltage is supplied to the end-cap electrodes in order to generate a mass selective resonant oscillation, which causes mass selective resonant ejection of the ions during a frequency varying mass scan. However, application of a dipole electric field along the z-axis of the ion trap is not the only way to achieve axial resonance excitation. In a paper titled “Modulation parametric resonances and their influence on stability diagram structure” (referred to hereinafter as Paper 2) published in the International Journal of Mass Spectrometry and Ion Processes, E. P. Sheretov gave the theory of ion excitation in a quadrupole electric field whereby any of its parameters such as frequency, amplitude and dc potential is modulated. This led the way to use of the quadrupole electric field alone, say by applying voltage to the ring electrode of quadrupole ion trap, to achieve ion trapping and sorting, as well as resonant excitation which may induce mass selective ion ejection.
Now, by means of ion optical simulation, the present inventor has discovered, inter alia, a practical method whereby mass scanning can be achieved solely by digital processing used to generate a rectangular wave drive voltage, obviating the need to supply a supplementary voltage to the ion trap device.
According to one aspect of this invention, there is provided a method for ejecting ions from a quadrupole ion trap including the steps of creating a digital control signal, using the digital control signal to control the timing of switching means to generate a time-varying rectangular wave voltage, supplying the time-varying rectangular wave voltage to the quadrupole ion trap to trap ions in a predetermined range of mass-to-charge ratio, and varying the duty cycle of every nth wave of the rectangular wave voltage (where n is an integer greater than unity) to cause ejection of trapped ions having a predetermined mass-to-charge ratio.
According to another aspect of the invention there is provided an apparatus for ejecting ions from a quadrupole ion trap including means for creating a digital control signal, switching means for generating a time-varying rectangular wave voltage in response to said digital control signal, the time-varying rectangular wave voltage being effective, when supplied to the quadrupole ion trap, to cause trapping of ions in a predetermined range of mass-to-charge ratio, and means for varying the duty cycle of every nth wave of the rectangular wave voltage (where n is an integer greater than unity) to cause ejection of trapped ions having a predetermined mass-to-charge ratio.
A mass analyser normally works in co-operation with an ion source. The ion source can be of the kind that generates ions directly inside the ion trap (e.g. a EI source) or of the kind that generates the ion species outside and then introduces them into the ion trap. Once the ions have been introduced into the ion trap, a high frequency voltage should be applied to the electrodes of the ion trap to trap these ions.
In FIG. 1, a high frequency digital control signal 1 is generated by a digital control unit 2. In this embodiment, the digital control unit 2 comprises a digital signal processor which may be in the form of a Direct Digital Synthesiser (DDS), a suitable filter and a comparator. The digital signal processor converts clock pulses into an analogue signal which is then subjected to smoothing by the filter. The comparator then compares the smoothed analogue signal with an adjustable threshold and generates the required digital control signal 1 as a result of the comparison. By this means, the timing accuracy of the digital control signal is much better than the period of the clock pulses and so very high frequency resolution of the digital control signal can be achieved. The control signal 1 is then supplied to a high voltage switch circuit 3 to generate the rectangular wave voltage. The high voltage switch circuit 3 includes switches 31 and 32 which are typically bipolar or FET transistors. The two switches 31,32 are connected together in series between a source 33 of a high DC voltage level (VH) and a source 34 of a low DC voltage level (VL). The switches 31,32 are alternately opened and closed in response to the digital control signal 1 so that when one switch is open the other is closed, and vice versa In this manner, the high and low DC voltage levels (VH,VL) are alternately supplied to an output 35 via the switches, thereby generating the time-varying rectangular wave voltage which is supplied to the ion trap. In this embodiment, the output 35 of the switch circuit 3 is connected to the ring electrode 4 of the ion trap and the two end cap electrodes are connected to ground or to a fixed voltage; alternatively, the time-varying rectangular wave voltage could be supplied to the end cap electrodes, the ring electrode being connected to ground or to a fixed voltage.
The high and low DC voltage levels (V1,V2) and the fixed voltage are expressed with respect to a common reference potential (in this case ground), and the fixed voltage can be used to provide a DC bias to offset any DC component U in the rectangular wave voltage, if required.
Application of the rectangular wave voltage to the ion trap causes a quadrupole trapping electric field to be formed inside the ion trap. The range of mass-to-charge ratios that can be trapped depends on different parameters of the rectangular wave voltage which may include a DC component U, an AC component V, frequency Ω=2πf, duty cycle d and r0, the radial dimension of the ion trap. For a standard quadrupole ion trap r0=√{square root over (2z 0)}, where z0 is the spacing of the end cap electrodes in the z-axis direction. In a paper titled “Ion Motion in the Rectangular Wave Quadrupole Field and Digital Operation Mode of a Quadrupole Ion Trap Mass Spectrometer” published in the Chinese Vacuum Science and Technology, V20 3, 2001, Li Ding analysed ion motion in the rectangular wave quadrupole field using the traditional a,q parameters which were previously used to study Mathieu's equation (although Mathieu's equation is no longer suitable for the rectangular wave quadrupole field). For ion motion in the z direction, these parameters are defined as                               a          z                =                  -                                    8              ⁢                                                           ⁢              e              ⁢                                                           ⁢              U                                      m              ⁢                                                           ⁢                              Ω                2                            ⁢                              r                0                2                                                                                                  q            z                    =                                    4              ⁢                                                           ⁢              e              ⁢                                                           ⁢              V                                      m              ⁢                                                           ⁢                              Ω                2                            ⁢                              r                0                2                                                    ,            where for a 50% duty cycle square wave, V is just the pulse height from low level to high level. FIG. 2 shows a stability diagram in which the stable region for motion in the z direction is shown shaded. In this stable region, ions oscillate in the z-direction with a limited amplitude and at certain oscillation frequencies and so are trapped within the ion trap. The intrinsic or secular frequency is the main frequency component of this oscillation, and was studied in paper 1 referenced above. In PCT/GB00/03964, a simplified expression for the case that the dc component U=0 was given as       ω    z    =            Ω              2        ⁢                                   ⁢        π              ⁢    arc    ⁢                   ⁢          cos      ⁡              [                              ch            ⁡                          (                                                                                          q                      z                                        2                                                  ⁢                π                            )                                ⁢                      cos            ⁡                          (                                                                                          q                      z                                        2                                                  ⁢                π                            )                                      ]            
As described in PCT/GB00/03964, by applying a voltage across the two end cap electrodes the trapped ions can be excited enhancing their movement in the z-axis direction. This is called dipole excitation. If the frequency of the dipole excitation voltage matches the intrinsic frequency of ion motion, resonance will occur and so ions with particular mass-to-charge ratio will undergo oscillatory motion which grows in amplitude in the z-axis direction with the result that those ions may be ejected through axial holes in the end cap electrodes. Mass analysis can thus be achieved by detecting these ejected ions while scanning either the rectangular wave drive frequency or the excitation frequency applied across the end cap electrodes, or both these frequencies in a fixed relation. This can be done digitally and has already been disclosed in PCT/GB00/03964.
The intrinsic oscillation can also be resonantly excited by application of an additional quadrupole field. In this case, an additional AC voltage can either be applied to the two end cap electrodes or superimposed on the driving rectangular wave voltage applied to the ring electrode. Because a quadrupole field accelerates ions in opposite directions on opposite sides of the ion trap with respect to the centre of the ion trap, resonance will occur if the frequency of this additional AC voltage is double the frequency of the intrinsic oscillation. This is clearly illustrated in FIG. 3 in which waveform (a) represents ion oscillation in the z-axis direction, waveform (c) shows a pulsed excitation voltage which is superimposed on the rectangular wave drive voltage (waveform (b)) applied to the ring electrode 4. As can be seen from FIG. 3 each excitation pulse gives rise to a defocusing force (shown by arrows in the drawing) causing oscillatory motion of ions in the z-axis direction to grow. FIG. 4 illustrates the effect of superimposing waveforms (b) and (c). In this illustration, the relative phases of the two waveforms are so chosen that every nth wave in the composite waveform (d) is slightly wider than the others, so increasing the duty cycle of that wave (in this example n=4), though clearly the superimposed waveforms may have a different phase relationship. Although the present invention embraces the foregoing technique involving the superimposition of separate waveforms (i.e. the time-varying rectangular wave drive voltage and a separate pulsed excitation voltage) waveform (d) of FIG. 4 can alternatively be derived, much more conveniently, directly from the digital control signal 2 alone, and this is the preferred technique. This approach obviates the need to generate an additional high voltage waveform which would then need to be superimposed on the rectangular wave voltage using additional circuitry.
In FIG. 3, the duty cycle of every 4th wave is increased in order to excite ions having an intrinsic frequency of             ω      z        =                  1        8            ⁢      Ω        ,and this corresponds to a,q parameters lying on line 1 in FIG. 2, which crosses the q axis at q1=0.269. Because the pulse excitation waveform contains higher order harmonic frequency components, oscillations at             ω      z        =                  1        4            ⁢      Ω        ,           ⁢            3      8        ⁢    Ω  will also be excited. In other words, this means that n−1 instability lines are created in the stability region when the duty cycle of every nth wave is modulated.
In order to avoid spurious peaks caused by these higher order frequency resonances during mass scanning, the frequency of the rectangular wave needs to be adjusted to ensure that all trapped ions have values of a,q to the left of the first resonance line 1 before a mass scan is started. During mass scanning the frequency of the rectangular wave voltage is gradually decreased and the duty cycle is varied. The amount of the variation of the duty cycle should be enough to eject an ion when it approaches resonance. This will depend on the speed of mass scan which in turn depends on the mass resolution required for the mass analysis. Normally the amount of variation       Δ    ⁢                   ⁢    d    dis smaller than 5%. FIG. 5 shows a computer simulation of a slow mass scanning process in which ions with different mass-to-charge ratios are ejected in sequence according to their mass-to-charge ratios. In this simulation, the duty cycle variation is only 2%. In order that all trapped ions are cooled down so that they occupy the middle of the ion trap before resonance ejection scanning starts, a buffer cooling gas may be introduced into the ion trap as part of the process. In the above simulation, He buffer gas at around 10−3 mbar pressure was taken into account.
The above embodiment only shows an example of this invention. In fact, there are many variants of the geometrical construction of a quadrupole ion trap. For example, the ion trap can be built to generate, as precisely as possible, the pure quadrupole electric field or to deliberately include high order electric fields (e.g. octupole field). It may be constructed using hyperboloid-shaped electrodes or a combination of flat and cylindrical-shaped electrodes. Also, the two end cap electrodes may be shaped and positioned asymmetrically, and differentially coupled to respective parts of the rectangular wave voltage. In this case, ions can be preferentially ejected from one side of the ion trap so that more ions will be detected by a charged particle detector placed on that side.
The main purpose of this invention is to carry out a mass scan in mass analysis, but using resonant ejection to dispel unwanted ions and retain the ions within a certain range of mass-to-charge ratio in the ion trap is also within the scope of this invention. Also the method disclosed herein can also be used in combination with, or assisted by, dipole excitation which can be easily achieved by applying a supplementary excitation voltage between the two end cap electrodes.
In the above illustration, the quadrupole ion trap is a rotationally symmetric ion trap, which is most commonly used. However, the ejection method can also be used with a linear quadrupole ion trap for the ejection of unwanted ions. In this case, the rectangular wave voltage is supplied to one pair of diagonally opposed electrodes and another pair of diagonally opposed electrodes is connected to a fixed potential or driven by a similar switch circuit which generates the rectangular wave voltage, but with reverse polarity. By suitably controlling the rectangular waveform shape, resonance along the x-direction and the y-direction can be made to happen at the same time or one after another.